Current human dependence on fossil or petroleum fuel as a limited resource has prompted development of alternative renewable energy sources. CO2 emitted from the burning of fossil fuel, as a green house gas and primary cause of global warming, adds further urgency. The United States, it is estimated, consumes more than 43 billion gallons per year of diesel fuel for transportation plus a multiple of this amount for gasoline and other oil-based fuels. In effort to provide fuel using sources other than so called fossil or petroleum derived fuels, biodiesel is derived from vegetable oil and animal fats. Corn based ethanol is now blended with gasoline at higher percentages. Current development of biofuels such as ethanol from corn requires extensive amounts of land and water. The use of food stuff in generation of transportation fuel has caused pricing pressure on agricultural commodities.
Photosynthesis converts the solar electromagnetic energy into stored chemical energy in long carbon chains by assembling carbon from CO2. Photosynthesis is the primary process on earth that sequesters and recycles CO2. Microalgae are the most photosynthetically efficient organisms. Algae, as a source of biofuel, have long been studied. The last energy crisis in the 1970's fueled research in alternative energy sources. A substantial amount of knowledge has been amassed by the U.S. Department of Energy's Aquatic Species Program. Algae may be grown in impaired water. Algae is highly efficient in photosynthesis with amazing rates of replication. Some algal strains can double every 4 to 6 hours. Algae when grown in certain conditions such as nitrogen-deficient culture can synthesize and accumulate fatty acids to levels greater than half of its dry weight. The algal fatty acids or oils are capable of being and currently is refined into jet fuel for the US Navy. However, cultivation systems allowing for scale up of algae culture from laboratory quantities to an industrial scale of production have heretofore been challenging. Conventionally, such algal cultivation systems can be separated into two categories: open vs. closed. Each conventional categories has pros and cons.
The open cultivation systems are “open” to the environment and the most common current form can be described as large raceway ponds. Such open ponds are the least expensive to build and operate. As such, open ponds was advocated by the ASP findings to compete with fossil fuel. However, raceway ponds require high water use due to constant evaporation. Open ponds offer sub-optimal light intensity control. Open ponds are prone to contamination from wild type strains overwhelming the desired cultured strains being propagated. Additionally, this type of open culture, being unprotected, is subject to predators which feed on algae. Large cultured ponds could be decimated in a few days by such predators. While large amount of resources are being funneled into designing or genetically modifying algae to improve yield and overcome the above-noted short comings of open ponds, it is unlikely that public opinion would allow the use of genetically altered mutant strains in open system with the accompanying risk of uncontrollable environmental contamination. Geographic limitations such as temperature and solar irradiance as well as land requirements are additional limitations.
Closed systems or photobioreactors are designed to address all of these limitations and concerns of the open pond systems to varying degrees. However, one of the major challenges is efficient utilization of solar irradiance. Solar irradiance as experienced at the earth's surface is highly variable, dependent of geophysical factors such as seasonal, daily and atmospheric variations. The phenomenon of “self shading or self shadowing” further complicates the utilization of solar irradiance. As light penetrates an algal culture, photons are absorbed by chlorophyll, decreasing the light intensity. This “self shading” is exaggerated in high cell concentration culture with high chlorophyll concentration. In fact, light does not penetrate very far at all in high cell concentration cultures, just a few millimeters. Optimal light intensity for algal photosynthesis has been demonstrated to be a small fraction of direct bright solar irradiance, in the range of 10%. An algal culture in an open pond commonly experiences a detrimental superficial culture layer in which the excessively high light intensity of direct solar beam causes photoinhibition, cell damage and possibly cell death. Through “self shading”, the high toxic level of light intensity is attenuated by chlorophyll absorption in the initial superficial “toxic” layer whereby a middle layer of culture experience “optimized” light intensity for algal photosynthesis. Any deeper layer of culture, as light intensity further attenuates, fails to receive sufficient light to drive algal photosynthesis.
Current art or culture systems, open or closed, rely on the strategy of cell movement in and out of the various conceptual light zones: 1) superficial toxic, 2) middle optimal and 3) deep deficient zones. Algal cells may move into the potentially toxic superficial layer to absorb photons for only milliseconds to microseconds before leaving the zone so damaging radicals do not build up. Current art of open ponds utilize large paddles to create stirring and current flow and typical closed systems such as tubular systems utilize pumps. Much of the current flow are laminar flow, parallel to the conceptual light zones described above, instead of more efficient turbulent flow in moving cells perpendicular through the light zones. Furthermore, algal movement through the pumps may experience shear injury at higher velocities. Despite the efficiencies gained with current closed systems, in general, photobioreactors are not cost effective to compete with fossil fuels in normal market conditions. The least expensive of the current closed systems are simple plastic bags with little structure. As these batch type cultures grow and chlorophyll content increases, larger and larger proportions of the culture in the middle does not receive enough light for photosynthesis, limiting achievable cell concentrations.
One prior art example of a closed system is that of U.S. Pat. No. 6,509,188 (Trosch) which teaches a photobioreactor having a reactor chamber formed of transparent material and having recesses and projections adapted to increase the reactor surface area with tubular projections and extensions. However, the Trosch patent construction does little to increase the structural integrity of the formed reactor panels, and lacks a narrow cross sectional area for limiting diffusion of nutrients to different height levels for the establishment of a nutrient gradient within the reactor.
As such, there exists an unmet need for algae culture systems which has a reasonable capital and operational cost to compete with crude oil under normal market conditions. Such a system should be able to continuously achieve an ultra-high cell concentration algal culture. Such a system should provide a structure and deployment thereof which provides a more even means to communicate solar and artificial radiation evenly and controllably. Further, such a system should inherently include means to prevent or limit bio-fouling or deposition of bio-film and should employ components which serve to expand surface area of the photobioreactor inner enclosure and concurrently provide a means for enhanced structural integrity. Additionally, such a system should employ a reactor which is structurally enhanced and which provides a stability of the desired high surface area to volume ratio of the algal culture as well as the desired small cross sectional area available which provides a means for limiting diffusion of nutrient to different height levels for establishing a nutrient gradient.